The golfer said he was in a bad place mentally after walking off the course on Friday midway through the second round of the Honda Classic.

He had completed only eight holes in the second round of the competition at Palm Beach, Florida, when he made his dramatic exit to the shock of playing partners Ernie Els and Mark Wilson.

Close to tears at the time, he told reporters in the car park shortly afterwards: "There's not much I can say, guys. I'm just not in a good place, mentally."

He had seemed distracted and in melancholy mood the previous day.

Only 48 hours earlier, his sports star girlfriend also crashed out of the Malaysian Open in what was one of the worst defeats of her career.

The former world No 1 was beaten by a competitor ranked 186th.

McIlroy apologised for his meltdown and blamed a toothache, despite being seen enjoying a sandwich a short while before he walked off the course, while Wozniacki claimed she was feeling unwell and had a high temperature.

The pair, who are usually very communicative with one another over Twitter, have not publicly conversed on the social networking site for several days.

Last week, the Danish beauty was questioned about her future with the Co Down golfer.

Asked if there were any marriage plans on the cards, she replied: "I think that question will be for him to answer."

When asked if she would accept if McIlroy proposed, she laughingly brushed off the question, saying: "This is getting personal!"

McIlroy is continuing what commentators are calling "The Curse of Nike" after signing a $250m (€192m) deal to switch from Titleist to the multi-sports giant's clubs in January.

His Dublin-based management company, Horizon Sports, quickly moved to quash speculation that his new clubs were to blame for his poor performance.

Earlier this week, Olympic gold medallist boxer Katie Taylor took a jab at Nike for its hat-trick of celebrity endorsement 'failures' in recent years while warning fans not to put their sports heroes on pedestals.

As the Oscar Pistorius murder case thunders on, Nike has been left reeling after yet another scandal involving one of its poster boys.

The double amputee South African known as 'Blade Runner' joined cyclist Lance Armstrong – who hit the headlines this year after admitting using performance-enhancing drugs – and serial sex cheat golfer Tiger Woods on the list of fallen idols.

Asked for his view on McIlroy's premature withdrawal last Friday from the Honda Classic, Tiger Woods said: "He's just got to be more ... just got to think about it a little bit more before he says something or does something."

Taylor, who's deeply religious, joked with journalists that perhaps the source of their troubles was "because they're sponsored by Nike and not Adidas" before warning sports fans not to elevate their heroes to God-like status.

"They're only human at the end of the day and people need to know that they can't put sports stars up on pedestals," she said.

"The Oscar Pistorius case is devastating for both families. With Lance Armstrong he just wanted to win medals more than anything else, but cheating is never acceptable.